This project aims to delineate immunologic responses to the human immunodeficiency viruses (HIVs), and to identify viral subfragments able to elicit protective immunity. We are investigating humoral and cellular immune responses to HIV-1 and HIV-2; studies with simian immunodeficiency virus are planned. We first focused on the humoral immune response, investigating HIV neutralizing antibody. Previous studies localized a major neutralizing epitope to the V3 loop of the viral gpl2O. Cross neutralization studies of a Zairian cohort using matched sera and viral isolates are being analyzed with regard to V3 loop sequences. The number of "serotypes" within a restricted geographic area will be determined. Results of these studies will assist design of a consensus epitope for elicitation of broadly reactive neutralizing antibody in future vaccines. Exploitation of in vitro immune selection has allowed us to pursue identification of alternate neutralizing epitopes and viral envelope regions important for infectivity. Extensive analysis of a single immune selected variant virus, resistant to neutralization by 30% of sera which neutralize the parental virus, has suggested that a point mutation in the transmembrane protein caused a conformational change, altering a neutralization epitope at another site. Selection and analysis of additional variants using the infectious molecular clones of the IIIB and MN prototype HIV-1 isolates and of the ISY HIV-2 isolate will allow delineation of additional viral envelope regions important in viral-host interactions. Assessment of the HIV-2 neutralizing antibody response in infected or immunized macaques is underway. This macaque model is also being used to identify viral epitopes which elicit cell mediated immunity. Cell proliferation assays and chromium release assays for cytotoxic T-lymphocytes are directed against HIV-2 gene products in vaccinia constructs as well as synthetic peptides representative of putative viral epitopes. Cloning of cytotoxic T-cells will facilitate mapping of epitopes which elicit protective cell mediated immune responses.